the solid crust has relatively low density, allowing it to rest on the fluid mantal

the upper mantle is 200-400*C

the earths crust is very thin compared to the diameter of the planet

the crust is made up of plates ranging from small to the size of a continent

continental crust is much thicker than oceanic crust

continental crust is made of less dense material than oceanic

the earths surface and its interior are constantly moving but you will never notice unless you are in an eathquake

beneth the earths crust lies the mantal, the upper part of which is solid

the crust and upper mantle are known as the lithosphere

below the lithosphere the mantle moves in convectional currents

the upper part of the mantle is called the asthenosphere

in the asthenosphere the flows affect the lithosphere above

because of the heat and pressure which builds up beneth the surface the crust is constantly being stressed which breaks it up

large scale processes within the earths crust are known as plate techtonics

when 2 plates are being pulled away from eachother deep cracks are opened in the crust

this allows magma to rise to the surface and then when it cools it forms new crust in the shape of a ridge

plate size can vary greatly from a few hundred to thousands of km across

the pasific and antartic plates are 2 of the largest

continental crust is composed of gannet rock which is low in density

oceanic crust is composed of basaltic rocks which are more dense

when oceanic crust meets continental crust it is the oceanic plate that is forced downwards

the difference in thickness of the two crusts are natures way of balancing the density of the 2 crusts

the continental crust can be as thick as 100km thick whereas the oceanic crust is usually only 7km thick

the convection currents in the mantle are driven by the heat of the core

heat is created by the pressure of the overlying materials and the radioactiveness of the core material

temperatures of the core are probably similar to those of the surface of the sun

the core is mostly made up of iron and nickle

the core is just over half the diameter of the earth

the core is 1 6th of the volume of the earth

the core is 1 3rd of the earths mass

the outer core is liquid

the inner core is solid

the currents in the outer core generate the earths magnetic feild

the 4 types of plate boundary between tectonic plates

there are 4 types of plate boundaries between tectonic plates - destructive, constructive, collision and conservative

destructive plate boundaries

distructive plate boundaries are found where two plates are moving together and oceanic plate is destroyed

they are associated with frequent earthquakes and volcanoes

the collision of 2 plates buckles the leading edge of the continental plate forming fold mountains and causing earthquakes

the oceanic plate is dragged downwards below the continental plate. this is known as subduction

magma rised through weaknesses in the continental crust forming volcanoes on the surface

constructive plate boundaries

these are found when new basaltic material rises to the surface, forcing plates appart

rising convection currents in the earths mantle cool and spread outwards as they near the surface this pulls the crust apart and creates fissures and faults through which molten magma can reach the surface

creation of new crust usually takes place in the sea forming a ridge and chains of submarine volcanoes. but sometimes these volcanoes reach the surface to form islands

these are found where 2 continental plates move towards each other. neither is destroyed but buckling takes place

earthquakes are common

because there is no suduction, no volcanoes are formed

the buckling has lead to the formation of the world biggest mountain range

conservative plate boundaries

these occur when plates are sliding past eachother

there is no plate being created or destroyed because there is no magma or subduction

the sliding is not smoth - there is friction between the 2 plates and extreme stresses build up in the crustal rocks when this is eventually relised it results in an earthquake

the size of the earthquake relates to the frequency of movement

on this boundry the plates are moving in the same direction but at different rates

some faults are visible on the surface but many are not

earthquakes and volcanoes

there is a close relationship between plate margins and tectonic activity

the hot spot theory is the theory that there are fixed spots in the mantle where magma rises to the surface. as the crust moves over these fixed spots volcanoes are created eventually forming chains of volcanoes.

there are many hot spots on the earth some of which have been extremely destructive in the past and pose threats for the planet in the future

there are many different types of volcano

experts have recognized 539 volcanoes that have erupted in recorded history these are classified as active

there are 529 volcanoes that have not erupted in historic times but exhibit clear evidence to do so again these are dormant

volcanoes that have not errupted in recorded history and show no signs of ever doing so are called extinct

in general the shape and structure of a volcano and explosive threat which they pose is related to the type of magma which created them

basaltic magma is usually found on constructive plate margins. it is high in temperature, very low in silica, with low gas content. this type of magma produces lava flow with relatively little explosive activity when it reaches the surface.

andesitic magma is formed at destructive margins where continental rocks are melted by rising magma. it is lower in temperature, has more silica and a lot more of dissolved gases. As a result this magma is much less fluid than basaltic magma and is more likely to explode when it reaches the surface.

the shape of volcanoes is a result of the type of magma that creates them and the frequency of their eruptions

volcanoes formed by magma that is basaltic tend to form very large gently sloping shapes known as shield volcanoes

volcanoes made up of anddesitic lavas tend to form composite volcanoes which are steep sided and make up of layers of lava and ash.

the ash is formed out of the material destroyed in the explosive eruptions which often blow the top of the mountain to pieces

pieces of as and rock are known as pyroclastics

the effects and impacts of volcanic hazards

earthquakes and volcanoes are good examples of tectonic hazards

not all hazards are equally devastating. the size and type of event is crucial

not all of earths inhabitants are at equal risk from natural hazards

unless people live close to a plate boundary it is very rare for them or their propety to be damaged by earthquakes - they are not vulnerable to the earthquake

if you do not live close to a volcano then you are not likely to be threatened by lava flows

even if you dont live near a volcano you can be affected my the clouds of volcanic ash which can significantly alter the climate of places miles, even continents, away from their point of origin.

capasity refers to the ability of a community to absorb and recover from the effects of a natural hazard

it is important to distinguish between the primary impacts and the secondary impacts of disasters

primary impacts are those that take place at the time of the event its self and are directly caused by it

secondary impacts are those that follow the event and are indirectly caused by it

the social and economic impacts of volcanic erruptions can be very considerable, and can include evacuation

management of tectonic hazards

the management of volcanic and earthquake hazards is expensive

the pattern of volcanoes and earthquakes are fairly well known and communities in the tectonically active areas can develop management strategies to cope

preparedness - being ready

preparation means that the government communities and individuals are ready to respond when disaster strikes and cope with the situation effectively

measures include the formulation of emergency plans, the development of warning systems and the training of personnel.

the measures may include evacuation plans for areas that may be at risk from a disaster and training for search and rescue terms

preparedness therefore encompasses those measures taken before a disaster event which are aimed at minimizing loss of life, disruption of critical services, and damage when the disaster occurs

mitigation - reducing the impact

mitigation measures are taken to reduce both the effect of the hazard and the vulnerability to it in order to reduce the scale of the disaster

they can be forced on the hazard itself or on the elements exposed to the threat

hazard specific measures include relocating people away from hazard prone areas and strengthening structures and using hazard resistant design to reduce damage when a hazard occurs

building design

one of the main ways of mitigating the impact of an earthquake is to improve the building design